1. Field of the Invention
The present invention relates generally to an apparatus and method for controlling a smart antenna in a mobile communication system, and in particular, to an apparatus and method for controlling signals provided to a smart antenna installed in a base station transceiver subsystem (hereinafter referred to as “BTS”) in a mobile communication system.
2. Description of the Related Art
In general, a code division multiple access (CDMA) mobile communication system distinguishes users through orthogonal codes, and performs communication between users and BTSs through mobile terminals and radio channels. In the BTS of the mobile communication system, a smart antenna is introduced to perform smooth communication with users over radio channels. A smart antenna technology has been developed to separately apply space vector weights to signals applied to each antenna element thereby forming a beam in a specific direction (beam forming), for signal transmission. In the CDMA mobile communication system, interference to neighboring terminals is reduced by transmitting data over forward channels using a smart antenna. This contributes to an increase in the power efficiency of a BTS.
A description will now be made of a CDMA mobile communication system with a smart antenna. When a user is located in the direction of +30° in front of a BTS and the number of antenna elements is 4, a beam pattern must be provided as illustrated in FIG. 1 in the ideal case. That is, a beam pattern of a signal must be generated in the direction of +30° in front of the BTS, so that a user terminal can correctly receive and restore the signal.
However, in the CDMA mobile communication system, when forward and reverse channels are established using a smart antenna, transmission signals may suffer phase change due to an error generated during initial installation of a base station system (BSS) or inaccuracy of a radio frequency (RF) device in operation and of hardware in the system. The phase change of transmission signals will be described herein below. Transmission signals may suffer phase variation due to performance variation during initial installation of a mobile communication system or due to heat or aging of equipment caused by continuous use of an RF device and hardware in the system. That is, although a beam pattern illustrated in FIG. 1 must be provided to transmit a good signal to a user terminal, a phase error illustrated in FIG. 2 by a sold line may occur due to the above-stated reasons. In FIG. 2, a curve ‘a’ shown by a dotted line indicates a beam pattern that must be actually transmitted, while a curve ‘b’ shown by a solid line indicates a beam pattern with a phase error.
When the actual transmission beam pattern is out of phase (or phase-distorted), if a BTS transmits a signal at normal transmission power, a user terminal has difficulty in correctly receiving the signal. Therefore, the BTS must transmit data at a higher power level, and this acts as a load to the system, causing a reduction of power that can be allocated to other terminals. In addition, the signal transmitted at higher power interferes with other terminals, causing a reduction in call quality.
Therefore, when a smart antenna is used, the phase variation must be compensated for. Reference will now be made to FIG. 3. In FIG. 3, a curve ‘a’ shown by a dotted line indicates an ideal beam pattern with a desired angle of 30°, a curve ‘b’ shown by a thin solid line indicates a beam pattern actually output from a BTS, and a curve ‘c’ shown by a bold solid line indicates a beam pattern provided for compensation. It is possible to pre-estimate an out-of-phase value (or phase distortion value) by calibrating a phase of the curve ‘b’ for an actually transmitted beam pattern and using a difference between the calibrated phase and a phase of the curve ‘a’ for the beam pattern that must be actually transmitted. As the phase distortion value can be pre-estimated, the BTS can pre-distort a phase by a predetermined value depending on the estimated phase distortion value, thereby transmitting a signal in accordance with the curve ‘c’. There is a method of transmitting a beam pattern of an actual transmission signal in accordance with the curve ‘a’ through the above-stated process.
In this method, phase distortion must be calibrated and compensated for in order to compensate for phase variation. There are four typical methods for calibrating phase distortion: (1) a pseudo-random noise (PN) code spreading method, (2) a Walsh code spreading method, (3) a method of using an orthogonal code and discrete Fourier transform (DFT) in a transmission path, and (4) a traffic signal feedback method.
A conventional process of calibrating a phase of a transmission signal will now be described with reference to FIG. 4. Referring to FIG. 4, a CDMA signal generator 100 generates transmission signals. The generated transmission signals are multiplied by a multiplier 120 by signals provided from an error calibration and compensation value calculator 180. Output signals of the multiplier 120 are added by an adder 140 to calibration signals provided from a calibration signal generator 110 via a switch 130, and then provided to an RF block 150. The RF block 150 converts the transmission signals into RF band signals, and transmits the converted RF band signals to antenna elements ANT0, ANT1, . . . , ANTn through a combiner 160. The combiner 160 feeds back part of combined signals of traffic signals and calibration signals to a switch 170. The feedback signals are applied to the error calibration and compensation value calculator 180 via the switch 170. The error calibration and compensation value calculator 180 calibrates errors of the feedback signals, calculates compensation values according to the calibrated errors, and provides the calculated compensation values to the multiplier 120. The multiplier 120 then multiplies the output signals of the CDMA signal generator 100 by the output signals of the error calibration and compensation value calculator 180. Through this process, an ideal beam pattern is formed by previously applying a predetermined distortion value illustrated in FIG. 3 to the phase-distorted signals illustrated in FIG. 2.
The PN code spreading method will be described with reference to FIG. 4. The PN code spreading method uses a PN code as a reference signal during transmission path calibration. That is, a reference signal generated by the calibration signal generator 110 becomes a PN signal. The reference signal is added to traffic signals by the adder 140, and the PN signal generated for the calibration functions as interference to the traffic signals, from the viewpoint of a terminal that receives the signal. That is, since the PN signal is not orthogonal with a Walsh code assigned to a traffic channel, the terminal cannot accurately extract only the calibration signal, resulting in a C/I (Carrier-to-Interference) loss.
For brief analysis, system environment will be simplified and then an influence of interference to a traffic signal for a calibration signal will be described. If it is assumed that a particular user firmly maintains orthogonality with other users in the cell, an interference signal includes only the reference signal used for calibration. When a signal, which is not orthogonal with a CDMA user and a control channel signal, is used as a reference signal, i.e., when the above-mentioned PN code spreading method is used, a carrier-to-interference ratio (CIR) is calculated as follows. If a data signal is defined as
            b      ⁡              (        t        )              =                  1                  2                    ⁢              (                              ±            1                    ±          j                )              ,a Walsh code as w(t) and a PN code as c(t), then a traffic signal can be represented byAb(t)w(t)c(t)cos(2πfct)  Equation (1)
In Equation (1), A is a constant having a dummy value.
A calibration signal, for which a PN spreading code is used, can be represented by A′c(t)cos(2πfct), and a terminal receives a signal defined asAb(t)w(t)c(t)cos(2πfct)+A′c(t)cos(2πfct)+n(t)cos(2πfct)  Equation (2)
In Equation (2), A′ indicates a level of a calibration signal, also having a dummy value, and n(t) indicates AWGN (Additive White Gaussian Noise). If c(t) is multiplied to despread Equation (2), then result becomesAb(t)w(t)cos(2πfct)+A′ cos(2πfct)+n(t)c(t)cos(2πfct)  Equation (3)
Equation (3) uses PN code property given byc(t)×c(t)=1  Equation (4)
As to conversion from Equation (2) into Equation (3), it is noted that formulas for traffic and calibration signals exclude c(t), and a noise is multiplied by c(t) to be spread. In addition, if a carrier frequency fc is multiplied in order to demodulate the signal of Equation (3), then the result becomes(Ab(t)w(t)cos(2πfct)+A′ cos(2πfct)+n(t)c(t)cos(2πfct))cos(2πfct)  Equation (5)
Equation (5) can be rewritten using trigonometric relations, as follows.
                                                                                                              (                                                                                            Ab                          ⁡                                                      (                            t                            )                                                                          ⁢                                                                                                  ⁢                                                  w                          ⁡                                                      (                            t                            )                                                                                              +                                              A                        ′                                                              )                                    ·                                                            cos                      2                                        ⁡                                          (                                              2                        ⁢                                                                                                  ⁢                        π                        ⁢                                                                                                  ⁢                                                  f                          c                                                ⁢                        t                                            )                                                                      +                                                      n                    ⁡                                          (                      t                      )                                                        ⁢                                      c                    ⁡                                          (                      t                      )                                                        ⁢                                                                          ⁢                                                            cos                      2                                        ⁡                                          (                                              2                        ⁢                                                                                                  ⁢                                                  π                          c                                                ⁢                        t                                            )                                                                                  ,                                                                          =                                                1                  2                                ⁢                                                      (                                                                                            Ab                          ⁡                                                      (                            t                            )                                                                          ⁢                                                  w                          ⁡                                                      (                            t                            )                                                                                              +                                              A                        ′                                            +                                                                        n                          ⁡                                                      (                            t                            )                                                                          ⁢                                                  c                          ⁡                                                      (                            t                            )                                                                                                                )                                    ·                                      (                                          1                      +                                              cos                        ⁡                                                  (                                                      2                            ⁢                                                                                                                  ⁢                            π                            ⁢                                                                                                                  ⁢                            2                            ⁢                                                          f                              c                                                        ⁢                            t                                                    )                                                                                      )                                                                                                          Equation        ⁢                                  ⁢                  (          6          )                    
In order to extract a baseband signal from the signal of Equation (6), the signal must be filtered by a low pass filter. The signal of Equation (6), after being filtered by the low pass filter, becomes a signal from which a carrier frequency component fc is removed, and can be written as
                              1          2                ⁢                  (                                                    Ab                ⁡                                  (                  t                  )                                            ⁢                              w                ⁡                                  (                  t                  )                                                      +                          A              ′                        +                                          n                ⁡                                  (                  t                  )                                            ⁢                              (                ct                )                                              )                                    Equation        ⁢                                  ⁢                  (          7          )                    n(t)(ct) must be changed to n(t)c(t) in below equation.
If Equation (7) is Walsh-demodulated using orthogonal property of a Walsh code, then it can be rewritten as
                              1          2                ⁢                  (                                    Ab              ⁡                              (                t                )                                      +                                          A                ′                            ⁢                              w                ⁡                                  (                  t                  )                                                      +                                          n                ⁡                                  (                  t                  )                                            ⁢                              w                ⁡                                  (                  t                  )                                            ⁢                              c                ⁡                                  (                  t                  )                                                              )                                    Equation        ⁢                                  ⁢                  (          8          )                    
A description will now be made of CIR. If CIR is defined as Equation (9), CIR by the PN code spreading method can be represented by Equation (10).
                    CIR        =                              I            C                                              I              OR                        +                          N              t                                                          Equation        ⁢                                  ⁢                  (          9          )                                        CIR        =                                                            A                2                            ⁢                              1                                  T                  s                                            ⁢                                                ∫                                      -                                                                  T                        s                                            2                                                                                                  T                      s                                        2                                                  ⁢                                                                            b                      2                                        ⁡                                          (                      t                      )                                                        ⁢                                      ⅆ                    t                                                                                                                        A                  2                                ⁢                                  1                                      T                    s                                                  ⁢                                                      ∫                                          -                                                                        T                          s                                                2                                                                                                            T                        s                                            2                                                        ⁢                                                                                    w                        2                                            ⁡                                              (                        t                        )                                                              ⁢                                          ⅆ                      t                                                                                  +                              N                t                                              =                                    A              2                                                      A                                  ′                  ⁢                                                                          ⁢                  2                                            +                              N                t                                                                        Equation        ⁢                                  ⁢                  (          10          )                    
In Equation (9), Ic denotes power of a carrier received at a mobile terminal, IOR denotes an interference in the same cell, and Ntdenotes noise power. Therefore, if it is assumed that interference in the cell includes only a calibration signal and a noise component is negligible, then CIR can be approximated as follows
                                                                                                              I                    C                                                                              I                      OR                                        +                                          N                      t                                                                      =                                                                                                    I                        C                                                                    I                        OR                                                                                    1                      +                                                                        N                          t                                                                          I                          OR                                                                                                      ≅                                                            I                      C                                                              I                      OR                                                                                  ,                                                                                                                                                      N                  t                                                  I                  OR                                            ⪡              1                                                          Equation        ⁢                                  ⁢                  (          11          )                    
It is noted from Equation (11) that a noise is multiplied by c(t) to be spread and its resultant level is reduced to a negligible value. Therefore, CIR is affected according to a level of calibration signal power. Therefore, when the PN code spreading method is used in the CDMA system, a C/I signal loss undesirably depends upon a power level of a PN signal, a calibration signal for calibrating phase distortion.
Second, the Walsh code spreading method will be described with reference to FIG. 4. The Walsh code spreading method performs calibration in the same way as the PN code spreading method, except that one of Walsh codes, is used as a calibration signal. That is, a Walsh code used in a CDMA system is used as the calibration signal output from the calibration signal generator 110. As a Walsh code used in the CDMA system is used as a calibration signal, orthogonality is provided between a calibration signal and a traffic signal. Thus, this method prevents the reference signal from functioning as interference to the traffic signal. However, the use of a Walsh code that can be used as a user code decreases the number of available Walsh codes, resulting in a reduction in system capacity. It is difficult to apply this method especially to a system requiring a high data rate, e.g., an EV-DO (Evolution Data Only) system, due to its limitation on available Walsh resources.
Third, the method of using an orthogonal code and DFT in a transmission path will be described. In this method, an m-sequence is used as a calibration signal. This method uses as many m-sequences as the number N of antenna elements, and provides a phase to each calibration signal by performing DFT on the m-sequences. Unlike in the other methods, an error between a calibration signal and a traffic signal is calibrated at a particular receiver separately prepared outside a terminal or a BTS. The receiver calibrates compensation information using autocorrelation property of the m-sequences and the DFT. That is, the receiver calibrates phase distortion by calibrating compensation information using the autocorrelation property of the m-sequences and the DFT, and then transmits the phase distortion value to a transmitter. In this method, since the receiver calculates an error and transmits the error to the transmitter, the receiver must include an addition device. That is, when the receiver is either separately provided outside the BTS or provided inside the BTS, a structure for calculating a phase distortion value using the autocorrelation property of the m-sequences and the DFT must be provided in addition to the structure of FIG. 4, causing an increase in the cost of the BTS.
Finally, the traffic signal feedback method will be described with reference to FIG. 4. This method feeds back a traffic signal rather than generating a reference signal and uses the feedback traffic signal as a reference signal, for transmission path calibration. Since no additional signal is used, this method does not have the defects that a reference signal functions as interference to a traffic signal. However, this method necessarily requires data storage for storing a traffic signal transmitted for comparison with a reference signal. That is, this method needs separate data storage for storing an output signal from the CDMA signal generator 100. In addition, the error calibration and compensation value calculator 180 synchronously reads the stored signal and an input signal, and determines phase distortion. In order to calculate phase distortion of each signal delivered to each antenna element, a switched power combiner must be used for a feedback traffic signal. That is, this method has a difficulty in simultaneously calibrating all antenna elements.
As described above, the PN code spreading method causes a C/I loss. The Walsh code spreading method reduces the number of available Walsh codes, causing a reduction in system capacity. Thus, the Walsh code spreading method can be hardly applied to the high-speed data transmission system such as the EV-DO system. The method of using an orthogonal code and DFT in a transmission path must include a separate device, causing an increase in system installation expense and an increase in the cost of a BTS. Finally, the traffic signal feedback method must further include a memory for storing signals transmitted to the BTS, and must synchronize the stored signals with input signals, increasing complexity of the circuit. In addition, this method cannot simultaneously calibrate phase distortion of all antenna elements.